It is a characteristic of most automatic focusing systems which are applied to opto/electronic imaging systems where a video signal is produced by scanning, that the high frequency content of the video signal is picked out (usually by filtering or by differentiation) and a signal indicating the resulting high frequency content used as an indication of the quality of focus of the image which is being scanned. Unfortunately electrical noise is inherently present in video signals to a greater or lesser degree and even in a well focused image the high frequency content may be of commensurate or even less amplitude than the electrical noise signal still present after filtering or differentiation. In this event it is virtually impossible to use the high frequency signal information and automatic focusing systems are therefore limited to applications where the high frequency content of the video signal is sufficiently great that it can be distinguished from electrical noise in the filtered video signal.
Scanning electron microscopes and similar instruments are usually required to work over a large range of signal to noise ratios and it is because of this that automatic focusing techniques have rarely met with much success when applied to such instruments. Whilst the automatic focusing systems can be made to operate on well defined high contrast images the signal to noise ratio of the signal obtained from typical images seen by such instruments is such as to prevent a good signal to noise ratio from being obtained so that it is difficult and usually impossible to obtain a focus indication signal by filtering or differentiation in the usual way.